1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus. More particularly, the present invention relates to a temperature control method for controlling in a stable manner a temperature of a fixing device built in an electrophotographic image forming apparatus and a fixing device having a constant temperature maintaining function.
2. Description of the Related Art
High performance electrophotographic image forming apparatuses are widely used for printing and have been extensively developed. The electrophotographic image forming apparatus performs printing by using a series of operations. The operations can include conduction, exposure, development, transferring, and fixing operations.
In the printing operation, charges generated by a conductor are formed on a surface of a photosensitive drum. A sheet of paper in a paper cassette is picked up and conveyed by a pick-up roller. Here, the surface of the photosensitive drum is exposed to a laser scanner unit (LSU), so that an electrostatic image can be formed on the surface. More specifically, the LSU emits laser beams corresponding to image data and directs them toward the surface of the photosensitive drum. In synchronization with the exposure operation, the paper sheet is transmitted to a transferring unit. The electrostatic image formed on the exposed surface of the photosensitive drum is developed by using toner supplied from a developing unit. As a result, the toner adhered on the surface of the photosensitive drum is transferred to the paper sheet. Next, the transferred paper sheet is transmitted to a fixing device having a heating roller and pressing roller. The fixing device fixes the toner on the paper sheet by using heat and pressure of the heating and pressing rollers. The toner-fixed paper sheet is fed out by a feeding roller, so that the printing operation for one paper sheet is completed.
The fixing device built in the electrophotographic image forming apparatus has a shape of tube. A heating lamp is provided as a heating device in an inner portion of the tube. In order to accurately perform the fixing operation, the fixing device must be maintained in a predetermined proper temperature. More specifically, the surface of the fixing device must be maintained in a proper target temperature in order to fix the toner on the paper sheet. The proper target temperature is called a printing temperature. The printing temperature depends on the fusing temperature of a developer and the thickness of the paper sheet used for the electrophotographic image forming apparatus.
In a conventional fixing device, a halogen lamp is used as the heating lamp. By turning on/off the halogen lamp, the temperature of the fixing device is increased up to or maintained in the proper target temperature for accurately fixing the toner on the paper sheet. More specifically, by turning on the halogen lamp, the temperature of the fixing device is increased from a room temperature to the target temperature. When the fixing device reaches the target temperature, the halogen lamp is turned off. Next, when the temperature of the fixing device decreases below the target temperature, the halogen lamp is turned on again.
FIG. 1 is a flowchart of a temperature control method for a conventional fixing device built in an electrophotographic image forming apparatus.
Firstly, it is determined whether or not a reference time for detecting the temperature of the fixing device has elapsed. The reference time denotes a time period for performing the temperature control operations for the fixing device. If the reference time elapses, it is determined whether or not the fixing device reaches a reference temperature. If the fixing device reaches the reference temperature, a heating lamp is turned off. If the fixing device does not reach the reference temperature, the heater is operated for a predetermined operating time. As shown in FIG. 1, in the conventional fixing device, the heating lamp is turned on and off by using unchangeable reference time and temperature.
FIG. 2 is a graph showing a change in temperature of the conventional fixing device built in the electrophotographic image forming apparatus.
If the temperature of the fixing device decreases below a target temperature TPT, the heating lamp is operated during a time interval (TI1, TI2) to increase the temperature of the fixing device. After that, the temperature of the fixing device continues to increase and exceeds the target temperature TPT, which is called overshoot. If overshoot occurs, the heating lamp is turned off to decrease the temperature of the fixing device. As a result, the temperature of the fixing device is allowed to decrease below the target temperature TPT. At this time, the heating lamp is turned on to increase the temperature of the fixing device. Due to the repeated heating and cooling of the fixing device, there occurs the so-called rippling, that is, a fluctuation of the temperature of the fixing device.
Unfortunately, the overshoot and rippling results in increasing power consumption and shortening the lifespan of the fixing device and its parts.
Therefore, in order to accurately fix the toner on the paper sheet, there is a need to stably control the temperature of the fixing device. In addition, there is a need to minimize the overshoot and rippling, which are affected by thermal conduction delay and thermal retention of a rubber layer of a fixing roller.